Turmeric-Derived Phenolic Compounds in Skin Protection
The role of curcuminoids, yellow phenolic compounds derived from turmeric roots (Curcuma longa, fam. Zingiberaceae), as topical antioxidants has been traditionally known in Asia and recently validated in laboratory experiments. Curcuminoids are reported to protect normal human keratinocytes from hypoxanthine/xanthine oxidase injury in in vitro studies, and they can protect the skin against a broad range of physical, chemical and biological factors injuring the skin1. Free radicals on the surface of the skin, generated through exposure to ultraviolet radiation, especially the UVB rays, chemicals or other environmental stress factors catalyze aging of the skin. Curcuminoids prevent free radical formation and scavenge free radicals in biological systems. This combined action was previously described as a Bioprotectant mechanism that protects the integrity of the living cell e.g. skin cell. The antioxidant effects of curcuminoids combined with their known inhibitory effects on cyclooxygenase 2 (Cox-2) render them useful as ingredients in anti-aging formulations and in topical formulations designed to maintain general skin health and integrity. Curcuminoids have also been found to inhibit the activity of tyrosinase, an enzyme that participates in melanogenesis, thereby preventing melanin formation with resultant lightening of the skin tone.2 
Tetrahydrocurcuminoids (THC) are color-free compounds derived from curcuminoids, the yellow, parent compounds of THC, in the process of hydrogenation. The process of hydrogenation of curcuminoids can also occur naturally in the gastrointestinal tract.3 The tetrahydrocurcuminoids"" similar biological properties to curcuminoids combined with the lack of yellow color, render them useful in achromatic food and cosmetic applications that currently employ conventional synthetic antioxidants.
Like the curcuminoids, THC have also shown significant antioxidant action in a number of in vitro and preclinical studies. 4-9 THC are valued as the ultimate metabolites of the curcuminoids in vivo. Several independent studies validated the significant antioxidant effects of the tetrahydrocurcuminoids and protection of skin against free radicals and UVB rays.